At a stage where a well has been drilled but not completed, it is often necessary to use explosive devices in the well. In open hole, it is necessary to lower a sidewall coring device which supports an explosive charge which fires a type of bullet or coring device into the wall to take a sample. This requires detonation of one or more explosives to take one or more samples. Alternatively, a well will be partially completed by placing a casing in the well borehole and cementing that casing in place. Later, one or more shaped charges form radially directed perforations into the casing and into the adjacent formations. This again requires detonation of explosive devices. Explosive devices are supported on carriers which are elongate cylindrical bodies which can support one or more explosive charges. It is not uncommon to place a very large number of shaped charges on a tool and lower them into the well borehole to form the perforations.
It is necessary to properly position all types of explosive devices at the right depth in the well borehole. This can be done in part by simply positioning the explosives and supporting cylindrical tool bodies on a cable. Cables, however, stretch and the amount of stretch in 10,000 feet of cable is very substantial. It might be predictable in a well which is precisely vertical. It is, however, difficult to know exactly where a 500 foot long assembly of shaped charges might be in a highly deviated well drilled from an offshore platform. The first few thousand feet of the well might be vertical, the next portion might deviate at sixty degrees from the vertical direction and thereby locate the shaped charges on a slant. Moreover, it may be necessary to incorporate safety devices including electronically operated interlocks as well as various navigational devices. All of this equipment includes expensive electronic devices as well as various and sundry pendulums for determining the vertical direction, solenoids for operation of the equipment, sensitive switches and the like.
Ordinarily, the devices described above are operated in a well which is filled at least partially, and perhaps almost totally, with the standing column of drilling fluid. This transmits the shock wave readily up and down the well borehole. The shock wave is substantial and can have a peak value of 100,000 pounds per square inch in the area where the explosive devices are included. When the explosion occurs, a pressure shock wave is propagated uphole and downhole. These shock waves are so severe that they can damage the more sensitive electronic devices which are included in the equipment.
The shock wave is ideally isolated from the more sensitive electronic components. The present disclosure sets out a shock isolation sub which is included in the string of tools which are lowered into the well borehole. This device serves to isolate the pressure shock wave from the explosives used in the operation. This device is especially useful in providing bidirectional isolation. It incorporates two separate and spaced apart stacks of shock wave reducing elements. Each stack is ideally made in similar fashion. However, one stack isolates motion in the upward direction and the other isolates motion in the downward direction. The stacks are made of alternate components. One component can be a simple metal washer, and the adjacent component can be something of a contrasting material such as a resilient material, i.e., rubber, various and sundry plastics, and other resilient materials. It is desirable to create a stack of alternating washers where adjacent members have different specific acoustical impedances to the propagation of the shock waves. This absorbs the shock wave energy and protects the electronic or other sensitive components which are located further along the tool from the shock wave source.
The present apparatus is particularly desirable in its operation in that a pressure balance across the seal areas is provided. When the shock wave is created, the peak loading is very substantial on a per unit area; this enables absorption of the shock wave without having to overcome the force created by an imbalance in seal areas. In other words, a pressure balance is achieved across the tool. As will be described in some detail, selected areas within the tool are deployed so that a pressure balance is obtained, and the balance between the respective areas keeps the tool in a fixed neutral position during detonation of such explosives, and the shock waves from the explosives can be propagated up to and through the shock sub but a substantial portion of the shock wave energy is absorbed by duplicate stacks of alternating washers. One stack is provided for shock waves which are traveling upwardly, and a similar but oppositely deployed stack handles shock waves traveling in the opposite direction. This enables the device to provide protection against the original shock wave and also against any reflected shock waves traveling along the device.
The present apparatus is briefly summarized as a shock sub which is provided with an axial passage to enable cables to extend through the shock sub. It is constructed with two specific areas which add cumulatively and which equal a larger area; the two areas are each one half of the larger area in the ideal embodiment. By proper control of applying pressure so that there is an atmospheric pressure and a hydrostatic pressure within the tool, and by appropriate movement of seals, the larger area is equal to the sum of the two small areas so that pressure imbalance across the tool is avoided and relative movement within the tool is thereby avoided. This enables the tool to transmit shock waves (traveling upwardly or downwardly) through the respective stacks of washers so that the shock waves are absorbed. Delicate electronic equipment which is otherwise located in the well borehole is protected.